chore: adopt Dart 3.9 toolchain — honest floor + tall-style reformat (T-353)

Raise the declared minimums in pubspec.yaml to what our deps already
require: Flutter >=3.35.0 / Dart >=3.9.0 (was 3.19.0 / 3.5.0). alchemist
0.12 needs Flutter 3.32; Dart 3.9 first ships in Flutter 3.35, so 3.35 is
the binding floor. Pin the exact build toolchain in .fvmrc (Flutter
3.44.1).

Moving to the Dart 3.9 language level switches `dart format` to the new
"tall" style and enables two new lints. This commit is the resulting
mechanical churn, isolated from any behaviour change:
  - whole-tree `dart format` reformat (tall style)
  - `dart fix` for unnecessary_underscores + use_null_aware_elements

No runtime behaviour change; `make test` green.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

test/widgets/src/clide_icon_test.dart

@@ -12,9 +12,7 @@ void main() {
     tearDown(() async => f.dispose());
 
     testWidgets('sizes a SizedBox + CustomPaint to the given size', (tester) async {
-      await tester.pumpWidget(
-        harness(f, const ClideIcon(FolderIcon(), size: 24)),
-      );
+      await tester.pumpWidget(harness(f, const ClideIcon(FolderIcon(), size: 24)));
       final sb = tester.widget<SizedBox>(find.byType(SizedBox).first);
       expect(sb.width, 24);
       expect(sb.height, 24);
@@ -22,9 +20,7 @@ void main() {
     });
 
     testWidgets('default color is globalForeground', (tester) async {
-      await tester.pumpWidget(
-        harness(f, const ClideIcon(CheckIcon())),
-      );
+      await tester.pumpWidget(harness(f, const ClideIcon(CheckIcon())));
       // Color is inaccessible after painting; ensure it renders without crashing.
       expect(find.byType(ClideIcon), findsOneWidget);
     });